Unpolarized light waves vibrate in a large number of planes about the axis of a light beam. If the light waves vibrate in one plane only, the light is said to be plane polarized. While several materials possess to a degree inherent polarizing properties, synthetic polarizing materials based on thin polymeric films are desirable for their comparative ease of manufacture and handling, their ability to be tailored for particular uses, and the comparative ease with which they may be incorporated into desired end products.
The production of linear light polarizing films has been well described in the art. Linear light polarizing films, in general, owe their properties of selectively passing radiation vibrating along a given electromagnetic radiation vector and absorbing electromagnetic radiation vibrating along a second given electromagnetic radiation vector to the anisotropic character of the transmitting film medium.
Dichroic polarizers are an absorptive variety of linear polarizers that owe their light-polarizing capabilities to the vectorial anisotropy of their absorption of incident light waves. The term “dichroism” is used herein to refer to the property of differential absorption of the components of an incident beam of light, depending upon the vibration directions of the components. Thus, light entering a dichroic film encounters two different absorption coefficients acting on light waves vibrating along different planes, one coefficient being low and one coefficient being relatively high. The emerging light vibrates predominantly in the direction of low absorption. One type of synthetic dichroic sheet polarizer is a polyvinyl alcohol-iodine complex polarizer and variants thereof, such as an “H-Sheet”-type polarizer or stained polarizer, the first such polarizer having been invented by Edwin H. Land of Polaroid Corporation and described in U.S. Pat. No. 2,454,515. In general, a polyvinyl alcohol-iodine complex polarizer comprises a light-absorptive linear polyiodide contained within a polyvinyl alcohol matrix. A polyvinyl alcohol-iodine complex polarizer is generally made, for example, by impregnating a film of polyvinyl alcohol or its derivative with an aqueous solution of a light-absorptive polyiodide or similar dichroic dye, and thermally stretching the film several times its length so that the resultant high molecular weight molecules are unidirectionally oriented. By orienting the polyvinyl alcohol matrix unidirectionally, the transition moments of the light-absorptive polyiodide become correspondingly oriented, and the material thus becomes visibly dichroic.
Another type of synthetic dichroic sheet polarizer is an intrinsic polarizer, such as a K-type polarizer. An intrinsic polarizer derives its dichroism from the light-absorbing properties of its matrix, not from the light-absorbing properties of dye additives, stains, or suspended crystalline material. Typically intrinsic polarizers comprise a sheet of oriented poly(vinyl alcohol) having an oriented suspension of a dehydration product of polyvinyl alcohol (i.e., polyvinylene). Intrinsic polarizers of this kind are formed by heating the polymeric sheet in the presence of a dehydration catalyst, such as vapors of aqueous hydrochloric acid, to produce conjugated polyvinylene blocks and unidirectionally stretching the polymeric sheet prior to, subsequent to, or during the dehydration step to align the poly(vinyl alcohol) matrix. By orienting the poly(vinyl alcohol) matrix unidirectionally, the transition moments of the conjugated polyvinylene blocks or chromophores are also oriented, and the material becomes visibly dichroic. A second orientation step or extension step may be employed after the dehydration step, as described in U.S. Pat. No. 5,666,223 (Bennett et al.).
An optical retarder modifies polarized light by retarding the optical path length for one of the orthogonal components of the light compared to the other orthogonal component. When the light emerges from the optical or phase retarder, there is a phase difference between the two orthogonal components of linearly polarized light. A circular polarizer or elliptical polarizer may be produced when an optical retarder is used in combination with a linear polarizer. Circularly polarized light is created when the two orthogonal components of linearly polarized light are phase shifted with respect to each other by λ/4, where λ represents the wavelength of the light. Elliptically polarized light results from an arbitrary phase shift between the two orthogonal components of incoming light. For example, a ray of unpolarized light passing through a linear polarizer becomes polarized in the polarization direction of the linear polarizer. When the polarization direction of the light is oriented 45 degrees with respect to the optical axis of the retarder, the resulting light is circularly or elliptically polarized, and the vibration direction of the polarized light ray after passing through the retarder appears to move in a helical pattern.